Ferrous metals are often used in environments where the surface is exposed to abrasive and corrosive elements. For example, in drilling oil wells, the wear surfaces of downhole earth boring equipment, such as rotary bits, drilling tools and the components thereof are exposed to the highly abrasive activity of drilling in an environment laden with corrosive elements, often at elevated temperatures.
The desire to create a hard outer surface to resist the abrasion and corrosion, while maintaining a ductile interior, has led to the science of case hardening of metals. In case hardening, a process is used which will produce a hard outer surface or case on a metal while permitting the core of the metal within to remain relatively soft and ductile when subject to normal ferrous metal through treatments.
One common process of case hardening is carburizing. Carburizing is particularly effective with low carbon and alloy steels and permits selective surface hardening of the metals. Carburizing consists of the process of diffusing nascent carbon into a ferrous surface at an elevated temperature. The depth of penetration of the nascent carbon depends upon the temperature and the length of time the ferrous material is exposed to a source of the carbon. The carbon can be supplied to the metal by a number of techniques. The ferrous surface can be exposed to a carbon rich gas or liquid. The material can also be surrounded by solid carburizing compounds to perform a pack carburization.
Ferrous metals have been treated with other materials than carbon. For example, ferrous metals have been exposed to a silicon rich environment which diffuses the silicon into the surface of the ferrous metal. This siliconization produces an outer case having both corrosion and wear resistance properties. Siliconization apparently produces a surface that is impervious to environmental attack as the iron atoms are no longer exposed at the surface.
Ferrous metals have also been treated with boron. If boron is diffused into the outer surface of a ferrous metal, the case becomes more resistant to corrosion with an improvement in the wear resistance of the outer surface resulting from the increase in hardness of the case surface.
Several attempts have been made to combine the desirable features of each case hardening material by hardening a ferrous substrate with multiple materials. For example, U.S. Pat. No. 3,923,348 issued to Peck on Dec. 2, 1975 discloses a technique for hardening a bushing. The bushing has a ferrite and martensite core. A carbon diffused layer is then provided on the ferrous substrate followed by a boron case. U.S. Pat. No. 3,922,038 to Scales issued on Nov. 25, 1975 discloses a treatment for ferrous substrates. In this technique, the ferrous surface is initially carburized. The ferrous material after carburizing is then boronized. Finally, the material is hardened and tempered. U.S. Pat. No. 4,188,242 to Scales issued on Feb. 12, 1980 also discloses a method of carburizing and boronizing steel with subsequent hardening and tempering.
Several shortcomings have been noted in the processes disclosed in these patents. The boron surface layer is extremely brittle and subject to cracking. While this problem can be somewhat alleviated by placing the boron layer in compression, this limits the applications of the process. In addition, it has been found that the boron case thickness must be maintained within a specified range to avoid cracking, restricting the versatility of the process. Therefore, a need exists for a process which combines the advantages of the various case materials which overcomes the shortcomings of the prior techniques.